The general feature of a conventional PCM electronic switching system, as shown in FIG. 1, consists of a plurality of subscriber lines (ICl-ICn), each of the subscriber lines including, SLIC (subscribe line interface circuits) 1, a telemetering signal eliminator 2, a telemetering signal mixer 3, an impedance conformer 4 and an integrated PCM-CODEC circuit 5 connected with a telemetering signal source 8 and a common bus 7 called a PCM highway whereby, a signal which is mixed with a telemetering signal in accordance with a synchronizing signal MSYNC from the telemetering signal source 8, is applied to the input of the SLIC 1 when call charge is counted up. And the integrated PCM-CODEC circuit 5, as shown in FIG. 2, includes a selector 11, a mixer 12, an transmitted-gain controller 13, an anti-aliasing filter (AAF) 14, a band pass filter (BPF) 15, a coder 16, a decoder 17, a low pass filter (LPF) 18, a received-gain controller 19, a power operational amplifier 20, a hybrid-balanced filter 21 and a gain controller 22.
In the conventional PCM system, the call charge is counted up by reversing the voltage of a battery between a tip T1 and a ring R1. In this case, unwanted clicking sound is generated. At this time, a sine wave of a frequency of 12 KHz or 16 KHz without a voltage reversal is used in order to remove the clicking sound. The telemetering signal (sine wave) of the frequency of 12 KHz or 16 KHz is mixed with a voice band signal; and then the mixed signal is transmitted to the tip T1 and the ring R1. But the problem is that if the mixed signal is coupled to the transmission path, the transmitter signal, as a result, will be modulated and distorted because the coupled signal is too large for processing. Accordingly, the coupled signal must be removed at the transmitting party. Considering in more detail therewith, as shown in FIG. 3, the current i1 flowing through the coil L1 is generated by a signal from the receiving terminal Rx, the terminal A0 of the integrated PCM-CODEC circuit 5, by means of a hybrid 2/4 converter. Thus, the secondary induced current i2 appears on the coil L2 coupled with a subscriber line, inducing a current i3 to flow through on the coil L3 coupled to the transmitting terminal Tx and, as a result, unwanted telemetering signal is applied to the transmitter terminal AI. Such results in that, in the conventional integrated PCM-CODEC circuit 5, the signal which is outputted from the terminal A0 whose frequency is lied on the voice frequency band is only compensated as the signal having the different polarity from the polarity of the induced current i3 which is coupled to transmitting terminal Tx by the hybrid-balanced filter 21, and the gain controller 22.
The conventional integrated PCM-CODEC circuit 5 can not eliminate a telemetering signal which is coupled from a receiving path. Consequently, additional circuit components are required for example, the telemetering signal eliminator 2 must be built around the transmission path of the integrated PCM CODEC circuit 5. In the operation of the conventional PCM system having the telemetering eliminator 2 and signal the mixer 3, the PCM-CODEC circuit 5 converts all input signals, containing even noise, into a PCM signal i.e., a pulse-code-modulation signal) in a tone frequency band. The coupled mixed signal (echo signal) from the receiving terminal Rx is evaluated to a value of the current i3 at the transmitting terminal Tx through the hybrid-balanced filter 21 and then mixed with a gain controlled signal of the gain controller 22 by the mixer 12, which results in reducing the weight of the unwanted mixed signal with the telemetering signal from the receiving terminal Rx to the transmitting Tx and enabling the applied-gain controller to adjust the variation of gain made through the telemetering signal eliminator 2 and the impedance conformer 4 as shown in FIG. 1. Sampling operation results in the replication of the spectrum of the tone signal, and (unless Nyquist's criterion is satisfied) introduces aliasing distortion which is removed by the anti-aliasing filter 14. After removing around the sampling frequency, the tone frequency signal is sampled and converted into the PCM signal through the coder 16 and then the PCM signal is transferred to the PCM highway of a common bus 7. If a telemetering signal eliminating means is not included in the PCM system, the voltage level of a signal of the telemetering signal source 8, in general, becomes more than several volts because the telemetering signal has high distortion rate in a public telephone network due to a well-known characteristic of the subscriber line and resultingly the telemetering signal induced on the transmitting terminal Tx has the voltage level of several hundred millivolts greater than that of a tone signal. And so the modulated and distorted tone signal appears in the integrated PCM-CODEC circuit 5. For this reason, it is very essential for the PCM system to employ the telemetering signal eliminator 2.
However, requirement of additional circuit elements, such as the telemetering signal eliminator 2, around the integrated PCM-CODEC circuit 5 makes the peripheral arrangement of the PCM system even more complicated. Because of the complexity of the peripheral arrangement and the increase of the number of subscriber lines on a unit board of a subscriber, the wiring of the unit board in the PCM system has been extremely difficult which often results in, a falling out in quality of the products.